City Layout
CNS vs PNS🏙️ The Nervous System: Two Districts
The nervous system has two main divisions. The Central Nervous System (CNS) is the city's control centre — the brain and spinal cord. The Peripheral Nervous System (PNS) is the road network — all the nerves connecting the CNS to every organ, muscle, and sense receptor in the body.
Information flows in: sensory signals travel TO the CNS. Instructions flow out: motor signals travel FROM the CNS to muscles and glands. Everything passes through the control centre.
🏛️ CNS — Central Nervous System
🧠 The Brain
- Cerebrum — largest part; conscious thought, memory, language, voluntary movement, sensory interpretation
- Cerebellum — coordination, balance, fine motor control
- Medulla oblongata — controls automatic (involuntary) functions: breathing, heart rate, swallowing
- Hypothalamus — thermoregulation, osmoregulation, hormonal control (links nervous + endocrine)
🔗 Spinal Cord
Pathway between brain and body. Also processes reflex arcs independently — without involving the brain. Protected by the vertebral column.
🛣️ PNS — Peripheral Nervous System
Somatic Nervous System
Controls voluntary movements — skeletal muscles you consciously move. Also carries sensory signals from skin, eyes, ears to CNS.
Autonomic Nervous System
Controls involuntary functions — organs, glands, smooth muscle. Has two branches:
- Sympathetic — "active" response: speeds heart rate, dilates pupils, slows digestion
- Parasympathetic — "rest and digest": slows heart rate, constricts pupils, stimulates digestion
| Brain Region | City Equivalent | Function |
|---|---|---|
| Cerebrum | City Hall — decision making | Thinking, memory, language, voluntary movement |
| Cerebellum | Traffic management system | Balance, coordination, smooth movement |
| Medulla oblongata | Automated systems (power grid) | Breathing, heartbeat, blood pressure — always on |
| Hypothalamus | Environmental monitoring station | Temperature, water balance, hunger, hormones |
| Spinal cord | Main highway + local bypass roads | Relays signals; processes reflex arcs locally |
Signal Network
How Signals Travel📡 Electrical Signals on Biological Cables
Nerve impulses are electrical signals — changes in charge across a neuron membrane. They travel at speeds of up to 120 m/s in myelinated neurons. The signal passes from neuron to neuron across tiny gaps called synapses, where chemical messengers (neurotransmitters) carry the signal across the gap.
😴 Resting Potential
- Inside of neuron membrane: negative (−70mV)
- Outside: positive (more Na⁺ ions outside)
- Na⁺/K⁺ pump maintains this difference
- Neuron is "charged up" and ready to fire
⚡ Action Potential (The Impulse)
- Stimulus causes Na⁺ channels to open
- Na⁺ rushes in → inside becomes positive (depolarisation)
- K⁺ channels open → K⁺ rushes out → repolarisation
- Na⁺/K⁺ pump restores resting potential
- This wave of depolarisation moves along the axon = impulse
🔬 Synapse Structure
- Pre-synaptic neuron — the sending neuron
- Synaptic cleft — tiny gap (~20nm) between neurons
- Post-synaptic neuron — the receiving neuron
- Pre-synaptic terminal contains vesicles filled with neurotransmitters
- Post-synaptic membrane has receptor proteins
⚙️ How It Works
- Impulse reaches pre-synaptic terminal
- Ca²⁺ enters → vesicles fuse with membrane
- Neurotransmitters released into synaptic cleft
- Neurotransmitters bind to receptors on post-synaptic membrane
- If threshold reached → new impulse generated
- Neurotransmitters broken down or reabsorbed
The Neuron
Three Types⚡ Neurons — The City's Communication Cables
A neuron is a specialised cell that carries electrical impulses. There are three types, each with a specific direction of travel in the signal pathway. Understanding which type does what is essential for understanding reflexes and voluntary responses.
👁️ Sensory Neuron — The Reporter
Structure
- Long dendron (carries impulse from receptor)
- Cell body positioned to the side of the axon
- Short axon leading to CNS
- Has receptor at one end — detects stimuli
Function
Carries impulses FROM receptors (sense organs, skin) TO the CNS. Direction: body → CNS. Always the first neuron in any nervous pathway.
🔄 Relay Neuron — The Switchboard Operator
Structure
- Short dendrites and axon
- Cell body in CNS
- Many connections to other neurons
- Found entirely within brain or spinal cord
Function
Connects sensory and motor neurons within the CNS. Processes and routes signals. Can connect to many other neurons — allows complex responses. Found only inside the CNS.
💪 Motor Neuron — The Dispatcher
Structure
- Cell body in CNS; long axon to effector
- Short dendrites at cell body
- Axon terminal forms neuromuscular junction with muscle
- Often heavily myelinated for speed
Function
Carries impulses FROM CNS TO effectors (muscles or glands). Direction: CNS → body. Always the last neuron in any nervous pathway. Triggers the response.
🔬 Myelin Sheath — The Fibre Optic Upgrade
Many neurons are wrapped in a myelin sheath — a fatty insulating layer formed by Schwann cells. Myelin speeds up impulse transmission dramatically by forcing the electrical signal to "jump" between gaps in the myelin (nodes of Ranvier) rather than travelling slowly along the whole membrane. This saltatory conduction increases speed up to 100×.
Reflex Arc
The Emergency Bypass🚨 The Emergency Bypass Road
A reflex is an automatic, involuntary response to a stimulus that does NOT require conscious thought from the brain. The signal travels through the spinal cord and back to the muscle — completely bypassing the cerebrum. This makes reflexes extremely fast — often completed before you're even aware the stimulus happened.
🗺️ The Reflex Arc Pathway
(e.g. sharp object)
(skin)
(to spinal cord)
(spinal cord)
(to muscle)
(muscle contracts)
(hand jerks away)
The brain receives the pain signal slightly AFTER the hand has already moved — awareness follows the reflex, not the other way around.
| Feature | Reflex Response | Voluntary Response |
|---|---|---|
| Involves brain? | No — spinal cord only | Yes — cerebrum processes and decides |
| Speed | Very fast (milliseconds) | Slower (conscious processing takes time) |
| Conscious control | None — automatic | Full — deliberate decision |
| Examples | Knee-jerk, pupil dilation, hand withdrawal from heat | Writing, speaking, choosing to move |
| Purpose | Rapid protection from harm | Complex, learned behaviour |
🎯 City Control Centre Check
Eight questions. Can you run the city?